The use of carbon fiber material, in general, is known in the field of composite materials.
A particular conventional example of carbon fiber material is a carbon fiber “fabric” including woven and unwoven forms that are produced as large sheets, frequently rectangular in shape. It is known conventionally to cut pieces having a desired shape (e.g., annular) from these sheets as part of the process of producing preforms. This approach has significant problems because the amount of carbon fiber fabric scrap waste generated is economically non-trivial.
Examples of various approaches for addressing the problem of scrap waste are seen in, for example, U.S. Pat. Nos. 4,955,123, 5,705,008, 5,686,117, 6,083,436, 5,952,075, 5,662,855, 5,546,880, and 5,113,568.
One of the approaches in the conventional art is to create spiral fabric (e.g., a braid, weave, or knit) having an internal structure that naturally lies smoothly in an annular shape due to its fabric structure. Spiral fabrics are also known in the art as polar weaves or polar braids. An advantageous reduction of scrap waste using spiral fabrics is at least somewhat offset by the cost and relative complexity of generating the spiral fabric having its unique fabric structure.
Another approach in the conventional art is to cut arcuate or trapezoidal segments from a carbon fiber sheet, wherein the cut segments are arranged in an annular shape to obtain a desired preform. However, this approach still generates a significant amount of scrap waste.
Another conventional approach is to use chopped or continuous fibers placed in an annular mold cavity. The fibers are bonded (for example, using a carbonizable resin) or are needled in a known fashion. In addition, a binder agent is sometimes used to help bind together the fibers. However, this approach also has certain problems.
First, the use of a binder agent is not desirable as it raises the likelihood of introducing impurities into the resultant preform and adds to the cost of manufacture.
Second, needling according to this approach is performed by rotating the annular preform material (either in or out of the annular mold) in an annular needle loom until the desired density characteristic is obtained. However, needling in this manner tends to suffer from “tracking.” Tracking is a phenomenon in which reciprocating needle members of the needle loom tend to land sequentially in the same spots across an extent of the fiber material, such that these spots are needled repeatedly. This creates a distinct (often visible) needling pattern in the material, thus the term “tracking.” When tracking occurs, the z-fiber content (i.e., the fibers reoriented by needling in the z-direction) in the region of the needle landings is disproportionately large. This can cause in-plane vs. out-of-plane thermal gradients and stress gradients that detrimentally affect the resultant material behavior.
In particular, tracking means that respective needle members of a needling device are landing at substantially the same spots with respect to the fiber material. This means that z-fiber transfer is essentially limited to certain spots in the fiber material, while other areas of the fiber material not beneficially reoriented by the needle members do not receive the effects of needling. In addition, tracking makes it difficult to attain desired material density or fiber content percentages so as to impart desirable physical characteristics.
It is noted that although the fiber material can be considered in terms of its density in a conventional sense (i.e., mass per volume), different types of fibers may be used in different articles, each type of fiber having non trivial differences in their respective densities. Thus, a comparison of densities of two different fiber articles (such as preforms) respectively made from different types of fibers (including among those disclosed above) may be unduly distorted by the differences in the densities of the underlying fibers.
For this reason, it is conventionally known in the art to consider the fiber content of a fibrous article, such as a preform, in terms of a dimensionless value, and moreover, as a percentage.
The fiber content percentage of a fibrous article is the density of the article divided by the density of the fiber, the result being expressed as a percentage. One of skill in the art will appreciate that this transformation essentially normalizes the variations in different fiber densities so as to provide directly comparable values.
In addition to the problem of tracking, the use of an annular loom is necessarily limited to operation on one preform at a time, which limits fabrication throughput.